(a) Field of the Invention
The present invention relates to electro-acoustic transducers, and more particularly, it pertains to an electro-acoustic transducer for use in loudspeakers, earphones or like devices, which uses, at least locally of its vibration system, a composite material having an extremely good acoustic characteristic.
(b) Description of the Prior Art
A known prior art electro-dynamic type speaker, in general, is constructed by a magnetic circuit system and a vibration system which is vibratably supported, by a suspension means, on a frame in such manner that its voice coil is positioned to lie in the magnetic gap of the magnetic circuit system. This vibration system is comprised of a diaphragm, a voice coil bobbin which is secured to the diaphragm, and a voice coil wound around the voice coil bobbin.
The material with which the diaphragm generally is made requires to be light in weight and to have a large E/.rho. (ratio between Young's modulus E and density .rho.), a large flexural rigidity E.multidot.I (wherein: E represents Young's modulus, and I represents second moment of section), and a large internal loss tan .delta.. More particularly, in a diaphragm, the larger the E/.rho. ratio is, the higher will become the resonance frequency, and accordingly the range of piston motion of the diaphragm will expand more. Thus, the frequency range of the speaker becomes broadened. Also, as E.multidot.I becomes greater, the distortions contained in the reproduced sound will accordingly decrease. Furthermore, as the internal loss tan .delta. increases, the value Q of the partial resonance of the diaphragm will decrease. Thus, it is possible to materialize flatness in the frequency characteristics of the diaphragm, i.e. it is possible to eliminate uneven colorification of the reproduced sound.
It is for the foregoing reasons that selection of constituent material of diaphragm becomes important. In the past, there has been used a paper sheet, or a thin film or foil made of a light metal having a large Young's modulus, such as aluminum (Al), boron (B), beryllium (Be), magnesium (Mg) or titanium (Ti), or a thin film of ceramics such as alumina (Al.sub.2 O.sub.3).
However, a paper sheet which is used in a diaphragm has the drawback that it has a small E/.rho. ratio.
In contrast thereto, a light metal as listed above, while having a relatively large E/.rho. ratio, has a very vmall internal loss tan .delta. of 0.01 or smaller. Thus, the overall internal loss of the whole diaphragm is small, causing a peak or a dip to appear in its frequency characteristic, and no desirable frequency characteristic can be obtained. Therefore, a diaphragm using such metal film or foil has the drawback that uneven colorification develops. On the other hand, a ceramic film has the advantage that it has a large E/.rho. ratio and its manufacturing cost is low, but it has problems in its processability and handling because of its fragility. For reasons stated above, each of these known materials has both strong points and weak points, and accordingly, it has not been possible to obtain a satisfactory diaphragm from the use of these materials.
Also, the material of a voice coil bobbin is required to be light in weight and to have such mechanical strength as will not develop deformation in itself during vibration. With respect specially to the mechanical strength of a voice coil bobbin, the selection of its material has become important in view of the recent increased demand for large output speakers.
In the past, paper sheet has been most widely used to realize voice coil bobbins. In view of the incapability of paper sheet to satisfy the abovesaid requirements, there have been proposed a voice coil bobbin which is made of various materials other than paper, such as a synthetic resin, e.g. polyamide resin, having an excellent resistance to heat, or light metal film or foil such as aluminum or duralumin.
However, a voice coil bobbin made of a synthetic resin is such that it has a too small Young's modulus and thus it lacks flexural rigidity to be used for the purpose of reproducing large outputs.
On the other hand, a voice coil bobbin made of a light metal film or foil such as aluminum or duralumin substantially satisfies the mechanical strength requirement. However, it has the big deficit that, in view of its being a good electric conductor, it gives rise to eddy current due to its vibration within the magnetic gap, and this, in turn, serves to work as a braking force to the vibration of the voice coil, with the result that the reproduced sound is adversely affected.